This work has been carried out primarily by high sensitivity differential scanning calorimetry (DSC). The DSC trace of Cp vs. T for undefatted human albumin exhibits two irreversible denaturation endotherms which we have shown to arise from the denaturation of albumin molecules containing low and high levels of long-chain fatty acids. Detailed studies of the effect of the stabilizers, caprylate and DL-N-acetyl-tryptophanate have been carried out. DSC studies of the denaturation of microtubules formed by capsid proteins of bacteiophage T4 show that the precursor state of the surface lattice composed of protein gp23 denatures at 65 C preceded by a minor endotherm at 45 C which is associated with no perceptible structural change in the particles when visualized by electron microscopy. The expanded surface lattice composed of protein gp23* (obtained upon proteolytic cleavage of gp23) denatures at 76 C with a greatly increased absorption of energy. The effect of incorporation o the proteins hoc and soc into the microtubular structure has been determined.